Cooling Pipes, Electrode Holders &amp; Electrode for an Arc Plasma Torch

ABSTRACT

A cooling tube for an arc plasma torch, comprising an elongate body with an end which can be arranged in the open end of an electrode, and a coolant duct extending therethrough, characterised in that at said end there is a bead-like thickening of the wall of the cooling tube pointing inwards and/or outwards, and an arrangement of a cooling tube for an arc plasma torch, comprising an elongate body with a rear end which can be releasably connected to an electrode holder of an arc plasma torch, and a coolant duct extending therethrough, and an electrode holder for an arc plasma torch, comprising an elongate body with an end for receiving an electrode and a hollow interior, and characterised in that on the outer surface of the cooling tube at least one projection is provided for centring the cooling tube in the electrode holder.

The present invention relates to cooling tubes, electrode holders and electrodes for an arc plasma torch, and also arrangements thereof and an arc plasma torch with them.

A plasma is the term used for an electrically conductive gas consisting of positive and negative ions, electrons and excited and neutral atoms and molecules which is heated thermally to a high temperature.

Various gases are used as plasma gases, such as mono-atomic argon and/or the diatomic gases hydrogen, nitrogen, oxygen or air. These gases are ionised and dissociated by the energy of an electric arc. The electric arc is constricted by a nozzle and is then referred to as a plasma jet.

The parameters of the plasma jet can be heavily influenced by the design of the nozzle and the electrode. These parameters of the plasma jet are, for example, the diameter of the jet, the temperature, the energy density and the flow rate of the gas.

In plasma cutting, for example, the plasma is constricted by a nozzle, which can be cooled by gas or water. In this way, energy densities of up to 2×10⁶ W/cm² can be achieved. Temperatures of up to 30,000° C. arise in the plasma jet, which, in combination with the high flow rate of the gas, make it possible to achieve very high cutting speeds on materials.

Because of the high thermal stress on the nozzle, it is usually made from a metallic material, preferably copper, because of its high electrical conductivity and thermal conductivity. The same is true of the electrode, though it may also be made of silver. The nozzle is then inserted into an arc plasma torch, called a plasma torch for short, the main elements of which are a plasma torch head, a nozzle cap, a plasma gas conducting member, a nozzle, a nozzle holder, an electrode with an electrode insert and, in modern plasma torches, a holder for a nozzle protection cap, and a nozzle protection cap. Inside the electrode, there is, for example, a pointed electrode insert made from tungsten, which is suitable when non-oxidising gases are used as the plasma gas, such as a mixture of argon and hydrogen. A flat-tip electrode, the electrode insert of which is made of hafnium, is also suitable when oxidising gases are used as the plasma gas, such as air or oxygen.

In order to achieve a long service life for the nozzle and the electrode, it is often cooled with a fluid, such as water, though it may also be cooled with a gas.

For this reason, a distinction is made between liquid-cooled and gas-cooled plasma torches.

In the state of the art, the electrode is made from a material with good electric and thermal conductivity, e.g. copper and silver or their alloys, and an electrode insert consisting of a temperature-resistant material, e.g. tungsten, zirconium or hafnium. For plasma gases containing oxygen, zirconium may be used. Because of its better thermal properties, hafnium is, however, better suited, since its oxide is more temperature-resistant.

In order to achieve a long service life for the electrode, the refractory material is introduced into the holder as an emission insert, which is then cooled. The most effective form of cooling is liquid cooling.

In the plasma torch, the arrangement with an electrode that is hollow in the interior and with a cooling tube inside it is known. In DD 87 361, for example, water flows through the interior of the cooling tube, streams against the bottom of the electrode and then flows back between the interior surface of the electrode and the exterior surface of the cooling tube.

The electrode often has a cylindrical or conical region extending inwards, with the cooling tube projecting beyond it. The coolant flows around this region and is intended to ensure a better exchange of heat between the electrode and the coolant.

Nevertheless, it repeatedly happens that when the apparatus is switched on for a long time, there is overheating at the electrode, which becomes apparent in the form of a considerable discoloration of the electrode holder and rapid burn-back of the electrode insert.

The invention is thus based on the problem of preventing, or at least reducing, overheating of the electrode of arc plasma torches.

According to the invention, this problem is solved by a cooling tube for an arc plasma torch, comprising an elongate body with an end that can be disposed in the open end of an electrode and with a coolant duct extending therethrough, characterised in that at said end there is a bead-like thickening of the wall of the cooling tube pointing inwards and/or outwards.

This problem is further solved by an arrangement of a cooling tube in accordance with any of claims 1 to 3 and an electrode having a hollow elongate body with an open end for arranging the front end of a cooling tube and a closed end, the bottom surface of the open end having a projecting region, over which the end of the cooling tube extends, and the thickening extends in the longitudinal direction over at least the projecting region.

In addition, this problem is solved by a cooling tube for an arc plasma torch, comprising an elongate body with a rear end that can be releasably connected to an electrode holder of an arc plasma torch and a coolant duct extending through it, characterised in that an external thread is provided for releasably connecting the rear end to an electrode holder, with a cylindrical outer surface adjoining this for centring the cooling tube relative to the electrode holder.

Furthermore, this problem is solved by an electrode holder for an arc plasma torch, comprising an elongate body with an end for receiving an electrode and with a hollow interior, characterised in that there is provided in the hollow interior an internal thread for screwing in a rear end of a cooling tube, with a cylindrical inner surface adjoining this for centring the cooling tube relative to the electrode holder.

This problem is further solved by an arrangement with a cooling tube according to any of claims 9 to 13 and an electrode holder according to any of claims 14 to 16, the cooling tube being screwed together with the electrode holder by means of the external thread and the internal thread.

In addition, the problem is solved by an arrangement of a cooling tube for an arc plasma torch, comprising an elongate body with a rear end that can be releasably connected to an electrode holder of an arc plasma torch and a coolant duct extending through it, and with an electrode holder for an arc plasma torch, comprising an elongate body with an end for receiving an electrode and with a hollow interior, characterised in that there is provided on the outer surface of the cooling tube at least one projection for centring the cooling tube in the electrode holder.

Furthermore, the present invention provides an electrode for an arc plasma torch, comprising a hollow elongate body with an open end for arranging the front end of a cooling tube therein and a closed end, the open end having an external thread for screwing together with the internal thread of an electrode holder, characterised in that adjoining the external thread, towards the closed end, there is a cylindrical outer surface for centring the electrode relative to the electrode holder.

In addition, the present invention provides an electrode holder for an arc plasma torch, comprising an elongate body with an end provided with an internal thread for receiving an electrode and with a hollow interior, characterised in that adjoining the internal thread, there is a cylindrical inner surface for centring the electrode relative to the electrode holder.

The present invention further provides an arrangement with an electrode according to any of claims 24 to 28 and an electrode holder according to any of claims 29 to 31, the electrode being screwed together with the electrode holder by means of the external thread and the internal thread.

According to a further aspect, this problem is solved by an arc plasma torch with a cooling tube according to any of claims 1 to 3 or 9 to 13, an electrode holder according to any of claims 14 to 16 or 29 to 31, an electrode according to any of claims 24 to 28 or an arrangement according to any of claims 4 to 8, 17 to 23 or 32 to 33.

In the cooling tube according to claim 1, it is advantageous for the thickening to extend over at least one millimetre in the longitudinal direction of the cooling tube.

The thickening conveniently leads to an increase in the external diameter by at least 0.2 millimetres and/or to a reduction of the internal diameter by at least 0.2 millimetres.

In the arrangement according to claim 4, it may be contemplated that it additionally comprises an electrode holder which has an elongate body with an end for receiving the electrode and with a hollow interior, wherein the cooling tube projects into the hollow interior and at least one projection is provided on the outer surface of the cooling tube for centring the cooling tube in the electrode holder.

It is convenient to provide a first group of projections arranged peripherally and spaced apart from one another.

In particular, it can be contemplated in this connection that they are arranged peripherally and spaced apart from one another, with the second group offset axially from the first group.

It is even more preferred for the second group of projections to be offset peripherally relative to the first group of projections.

The cooling tube according to claim 9 may be provided with a stop face for fixing the cooling tube axially in the electrode holder.

It is advantageous for the cylindrical outer surface to have a peripheral groove.

In particular, an O-ring may be disposed in the groove for sealing purposes.

According to a particular embodiment of the invention, the cylindrical outer surface has an external diameter which is exactly the same size as or larger than the external diameter of the external thread.

In the electrode holder according to claim 14, it is convenient to provide a stop face for fixing the cooling tube axially in the electrode holder.

It is advantageous for the cylindrical inner surface to have an internal diameter which is exactly the same size as or larger than the internal diameter of the internal thread. The principle applicable here is D6.1=(D.61 a−D6.1 i)/2 (“a” indicating external and “i” indicating internal).

In accordance with a particular embodiment of the arrangement according to claim 17, the cooling tube and the electrode holder are designed such that towards the front end, there is an annular gap between them.

In addition, it is conveniently contemplated that the cylindrical outer surface of the cooling tube and the cylindrical inner surface of the electrode holder have narrow tolerances relative to one another.

In the arrangement according to claim 20, it is convenient to provide a first group of projections arranged peripherally and spaced apart from one another. In particular, exactly three projections may be provided, which are preferably arranged to be offset from one another by 120°.

In addition, a second group of projections may be provided, arranged peripherally and spaced apart from one another, with the second group offset axially relative to the first group. The second group of projections may likewise consist of exactly three projections, which are preferably arranged to be offset from one another by 120°.

The second group of projections is advantageously offset peripherally relative to the first group of projections. The offset may be 60°, for example.

In the electrode according to claim 24, it is convenient to provide a stop face for fixing the electrode axially in the electrode holder.

In particular, the cylindrical outer surface may have a peripheral groove with an O-ring disposed in it for sealing purposes.

According to a particularly advantageous embodiment, the cylindrical outer surface has an external diameter which is exactly the same size as or larger than the external diameter of the external thread.

In the electrode according to claim 29, a stop face may be provided for fixing an electrode axially in the electrode holder.

It is advantageous for the cylindrical inner surface to have an internal diameter which is exactly the same size as or larger than the internal diameter of the internal thread. The principle applicable here is D6.4=(D6.4 a−D6.4 i)/2.

In the arrangement according to claim 32, it is advantageous for the cylindrical outer surface of the electrode and the cylindrical inner surface of the electrode holder to have narrow tolerances relative to one another. It is customary here to use a so-called transition fit, meaning, for example, an outer tolerance: 0 to −0.01 mm, and an inner tolerance: 0 to +0.01 mm

The invention is based on the surprising finding that the thickening causes the gaps between the cooling tube and the electrode to become narrower, but without reducing the cross-section in the rear region of the arc plasma torch head. In this way, a high flow speed of the coolant is achieved at the front, between the cooling tube and the electrode, which improves the heat transfer.

The heat transfer is additionally or alternatively improved by suitably centring components of the plasma torch head.

The invention is based on the finding that the heat transfer between the electrode and the coolant is not ideal. In this connection, the pressure, the flow speed, the volume flow and/or the pressure differential of the coolant in the flow path may not be adequate in the front region, in which the cooling tube projects beyond the inwardly extending region of the electrode. In addition, the problem has been recognised that the annular gap between the electrode and the cooling tube may differ in size on its circumference if it is not centrally positioned. This results in an uneven distribution of the coolant around the inwardly extending region of the electrode. This impairs the cooling.

Further features and advantages of the invention will become clear from the enclosed claims the following description, in which four embodiments are illustrated in detail with reference to the schematic drawings. There,

FIG. 1 shows a longitudinal sectional view through a plasma torch head in accordance with a first particular embodiment of the present invention;

FIG. 2 shows an individual view of a cooling tube of the plasma torch head shown in FIG. 1, seen from above (left) and in a longitudinal sectional view (right);

FIG. 3 shows details of the connection between the electrode and the electrode holder in a longitudinal sectional view of the plasma torch head shown in FIG. 1;

FIG. 4 shows details of the electrode holder shown in FIG. 3, partially in a longitudinal section;

FIG. 5 shows details of the connection between the electrode holder and the cooling tube of the plasma torch head shown in FIG. 1;

FIG. 6 shows details of the electrode holder shown in FIG. 5, partially in a longitudinal sectional view;

FIG. 7 shows a detail (section A-A) of the connection between the electrode holder and the cooling tube of the plasma torch head shown in FIG. 1;

FIG. 8 shows an individual illustration of the electrode of the plasma torch head shown in FIG. 1, in a longitudinal sectional view;

FIG. 9 shows a longitudinal sectional view through a plasma torch head in accordance with a second particular embodiment of the present invention;

FIG. 10 shows an individual view of a cooling tube of the plasma torch head shown in FIG. 9, seen from above (left) and in a longitudinal sectional view (right);

FIG. 11 shows details of the connection between the electrode holder and the cooling tube of the plasma torch head shown in FIG. 9;

FIG. 12 shows a longitudinal sectional view through a plasma torch head in accordance with a third particular embodiment of the present invention;

FIG. 13 shows an individual view of a cooling tube of the plasma torch head shown in FIG. 12, seen from above (left) and in a longitudinal sectional view (right);

FIG. 14 shows details of the connection between the electrode holder and the cooling tube of the plasma torch head shown in FIG. 12;

FIG. 15 shows a longitudinal sectional view through a plasma torch head in accordance with a fourth particular embodiment of the present invention;

FIG. 16 shows an individual view of a cooling tube of the plasma torch head shown in FIG. 15, seen from above (left) and in a longitudinal sectional view (right); and

FIG. 17 shows details of the connection between the electrode holder and the cooling tube of the plasma torch head shown in FIG. 15;

FIG. 1 shows a first particular embodiment of a plasma torch head in accordance with the present invention; Said plasma torch head has an electrode 7, an electrode holder 6, a cooling tube 10, a nozzle 4, a nozzle cap 2 and a gas line 3. The nozzle 4 is fixed in place by the nozzle cap 2 and a nozzle holder 5. The electrode holder 6 receives the electrode 7 and the cooling tube 10 via a thread in each case, namely the internal thread 6.4 and the internal thread 6.1. The gas line 3 is located between the electrode 7 and the nozzle 4 and causes a plasma gas PG to rotate. In addition, the plasma torch head 1 has a secondary gas protection cap 9, which in this embodiment is screwed onto a nozzle protection cap holder 8. A secondary gas SG, which protects the nozzle 4, especially the nozzle tip, flows between the secondary gas protection cap 9 and the nozzle cap 2.

The cooling tube 10 (see also FIG. 2) is attached to the rear part of the electrode holder 6, and the electrode 7 is attached to the front part of the electrode holder 6. The cooling tube 10 projects beyond a region 7.5 of the electrode 7 extending inwardly, i.e. away from the nozzle tip (see also FIGS. 3 and 8). In that region, the internal diameter D10.8 over the length L10.8 of the cooling tube 10 is smaller than the internal diameter D10.9 of the internal portion 10.9 of the cooling tube 10 facing backwards, and the external diameter D10.10 over the length L10.10 of the cooling tube 10 is larger than the external diameter D10.11 of the external portion 10.11 of the cooling tube 10 facing backwards. This thus gives rise to a bead-like thickening 10.18 of the wall 10.19 of the cooling tube, facing inwards and outwards. This ensures that the flow cross-section available to the coolant is only constricted in the front internal portion 10.8 and front external portion 10.10, in which a high flow velocity of a coolant is required for good heat dispersal, and the greatest possible flow cross-section is available in the rear region in order to keep the pressure drops in the rear internal portion 10.9 and rear external portion 10.11 as low as possible. A coolant first flows in the flow path through WV1 (water supply line 1) into the interior of the cooling tube 10 and encounters the inwardly extending region 7.5 of the electrode 7, before flowing back via the flow path WR1 (water return line 1) in the space between the cooling tube 10 and the electrode 7 and electrode holder 6.

The plasma jet (not shown) has its point of attack on the outer surface of an electrode insert 7.8. That is where the most heat arises, which has to be dissipated in order to ensure a long service life of the electrode 7. The heat is conducted via the electrode 7 made from copper or silver to the coolant in the interior of the electrode.

In the region in which the cooling tube 10 projects beyond the inwardly extending region 7.5 of the electrode 7, the gap between the opposing surfaces of the front internal portion 10.8 of the cooling tube and the electrode region 7.5 of the electrode 7 and of the front external portion 10.10 and the inner surface 7.10 of the electrode is very small. It is in the region of 0.1 to 0.5 mm.

In addition, coolant flows in the space between the nozzle 4 and the nozzle cap 2 via a flow path WV2 (water supply line 2) and WR2 (water return line 2).

As is also illustrated in FIGS. 5 and 6, the cooling tube 10 is screwed to the electrode holder 6 via the external thread 10.1 and the internal thread 6.1. The cooling tube 10 and the electrode holder 6 are centred relative to one another by means of the cylindrical outer surface 10.3 of the cooling tube 10 and the cylindrical inner surface 6.3 of the electrode holder 6. These have narrow tolerances relative to one another in order to achieve good centring. In this context, the tolerance of the cylindrical outer surface 10.3 can be the nominal size of the external diameter D10.3 from 0 to −0.01 mm and the tolerance of the cylindrical inner surface 6.3 can be the nominal size of the internal diameter D6.3 from 0 to +0.01 mm. The internal thread 6.1 of the electrode holder 6 and the external thread 10.1 of the cooling tube 10 have sufficient play relative to one another, so that the cooling tube 10 can easily be screwed into the electrode holder 6. It is only just before tightening that the centring occurs by means of the cylindrical inner surface 6.3 and cylindrical outer surface 10.3, which have narrow tolerances and face each other in the screwed-in state.

The external diameter D10.3 of the cylindrical outer surface 10.3 of the cooling tube 10 is at least the same size as or larger than the external diameter D10.1 of the external thread 10.1.

The internal diameter D6.3 of the cylindrical inner surface 6.3 of the electrode holder 6 is larger than the minimum internal diameter D6.1 of the internal thread 6.1, where D6.1=(D6.1 a−D6.1 i)/2.

The centring described above ensures the parallel alignment of the cooling tube 10 to the axis M of the plasma torch head 1, a uniform annular gap between the cooling tube 10 and the electrode region 7.5 and thus a uniform distribution of the coolant flow in the electrode interior, especially in the region of the front portion 10.8 of the cooling tube 20 and of the inwardly extending electrode region 7.5. When screwed in tightly, the stop faces 10.2 and 6.2 rest on one another. This causes the cooling tube 10 to be fixed axially in the electrode holder 6.

As is also illustrated in FIGS. 3 and 4, the electrode 7 is screwed to the electrode holder 6 by means of the external thread 7.4 and the internal thread 6.4. The electrode 7 and the electrode holder 6 are centred relative to one another by means of the cylindrical outer surface 7.6 of the electrode 7 and the cylindrical inner surface 6.6 of the electrode holder 6. The outer surfaces have narrow tolerances relative to one another in order to achieve good centring. In this context, the tolerance of the cylindrical outer surface can be the nominal size of the external diameter D7.6 from 0 to −0.01 mm and the tolerance of the cylindrical inner surface 6.3 can be the nominal size of the internal diameter D6.6 from 0 to +0.01 mm. The internal thread 6.4 of the electrode holder 6 and the external thread 7.4 of the electrode 7 have sufficient play relative to one another, so that the electrode 7 can easily be screwed into the electrode holder 6. It is only just before tightening that the centring occurs by means of the cylindrical surfaces 6.6 and cylindrical outer surface 7.6, which have narrow tolerances and face each other in the screwed-in state.

The external diameter D7.6 of the cylindrical outer surface 7.6 of the electrode 7 is at least the same size as or larger than the maximum external diameter D7.4 of the external thread 7.4 (see FIG. 8).

The internal diameter D6.6 of the cylindrical inner surface 6.6 of the electrode holder 6 is larger than the internal diameter D6.4 of the internal thread 6.4, where D6.4=(D6.4 a−D6.4 i)/2.

The centring described above is necessary for the parallel alignment of the electrode 6 to the axis M of the plasma torch head 1, which in turn ensures a uniform distribution of the coolant flow in the electrode interior, especially in the region of the front internal portion 10.8 of the cooling tube 10 and of the inwardly extending region 7.5 of the electrode 7. The purpose of centring the electrode 7 relative to the electrode holder 6 is to secure the centricity relative to the other components of the plasma torch head, especially the nozzle 4. The latter serves to form a uniform plasma jet, which is partly determined by the positioning of the electrode insert 7.8 of the electrode 7 relative to the nozzle bore 4.1 of the nozzle 4. In addition, the cylindrical outer surface 7.6 has a groove 7.3 with an O-ring 7.2 disposed in it for sealing purposes. When screwed in tightly, the stop faces 7.7 and 6.7 rest on one another. This causes the electrode 7 to be fixed axially in the electrode holder 6.

A further improvement in the radial centring of the cooling tube 10 relative to the electrode holder 6 is obtained by means of a group of projections 10.6 and a group of projections 10.7, which are located on the outer surface of the cooling tube 10. They fix the distance from the inner surface of the electrode holder 6. In this embodiment, there are three projections 10.6 and 10.7 per group distributed offset by 120° on the periphery of the outer surface of the cooling tube and also with an offset L10 a in the longitudinal direction of the cooling tube 1 relative to one another (see FIGS. 2 and 7). The projections 10.6 are arranged in this case offset by 60° relative to the projections 10.7. This offsetting improves the radial centring. At the same time, the projections 10.7 can be used as a counterpart for a tool (not shown) for screwing the cooling tube 10 in and out. The projections 10.6 and 10.7 have a rectangular cross-section when seen from the front region 10.8. This means that only the corners of the rectangular cross-sections rest on the cylindrical inner surface 6.11 of the electrode holder 6. In this way, a high degree of centricity is achieved, while at the same time preserving ease of assembly.

FIG. 9 shows a further particular embodiment of a plasma torch head 1 in accordance with the invention, which differs from the embodiment shown in FIGS. 1 to 8 in the design of the front internal portion 10.8 of the cooling tube 10 (see also FIG. 10). The length L10.8 of the internal portion 10.8 is shorter, as a result of which the flow cross-section is increased considerably only in the front-most region. The lengths of the front internal portion 10.8 and the front external portion 10.10. are identical here. In addition, in the region in which the electrode holder 6 and the cooling tube 10 are screwed together, there is a groove 10.4 in the cylindrical outer surface 10.3 of the cooling tube 10, with an O-ring 10.5 disposed in the groove for sealing purposes (see also FIG. 11).

FIG. 12 shows a further particular embodiment of a plasma torch head of the invention, which differs from the two embodiments shown in FIGS. 1 to 11 in the design of the front internal portion 10.8 of the cooling tube 10 (see also FIG. 13). The length L10.8 of the internal portion 10.8 is shorter than in FIG. 1, and the length L10.10 of the front external portion 10.10 is greater than in FIG. 9. As a result, the flow resistance of the overall arrangement is reduced, since narrow gaps are only found in the front-most part between the cooling tube and the electrode.

The centring between the cooling tube 10 and the electrode holder 6 is likewise achieved by means of a cylindrical inner surface 6.3 and a cylindrical outer surface 10.3. These are, however, arranged differently from what is shown in FIGS. 1 and 9. As a result of this arrangement, the cylindrical centring surfaces are enlarged. This further improves the centring and is achieved by changing the order “thread—centring surface—stop face” to “thread—stop face—centring surface”. A further advantage is that the size of the unit is not increased. If the order were retained, the stop face would have to have a different diameter from the centring surface.

FIG. 15 shows a further special embodiment of the plasma torch head of the invention. It differs from the embodiment of FIG. 1 in the design of the front internal portion 10.8 of the cooling tube 10 (see also FIG. 16). The lengths of the front internal portion 10.8 and the front external portion 10.10. are identical here. In their length, said portions correspond to the region 7.5 of the electrode 7.

Centring between the cooling tube 10 and the electrode holder 6 is achieved as in FIG. 12. In addition, in the region in which the electrode holder 6 and the cooling tube 10 are screwed together, there is a groove 10.4 in the cylindrical outer surface 10.3 of the cooling tube 10, with an O-ring 10.5 disposed in the groove for sealing purposes. That is illustrated in FIG. 17.

The features of the invention disclosed in the present description, in the drawings and in the claims can be essential to implementing the invention in its various embodiments both individually and in any combinations. 

1. A cooling tube (10) for an arc plasma torch, comprising an elongate body (10.13) with an end (10.17) which can be arranged in the open end (7.12) of an electrode (7) and a coolant duct (10.15) extending therethrough, characterised in that at said end (10.17) there is a bead-like thickening (10.18) of the wall (10.19) of the cooling tube (10) pointing inwards and/or outwards.
 2. The cooling tube (10) as claimed in claim 1, characterised in that the thickening (10.18) extends over at least one millimetre in the longitudinal direction of the cooling tube (10).
 3. The cooling tube (10) as claimed in claim 1, characterised in that the thickening (10.18) leads to an increase in the external diameter (D10.11) by at least 0.2 millimetres and/or to a reduction of the internal diameter (D10.9) by at least 0.2 millimetres.
 4. The arrangement of a cooling tube (10) in accordance with claim 1 and an electrode (7) having a hollow elongate body (7.11) with an open end (7.12) for arranging the front end (10.17) of a cooling tube (10) and a closed end (7.13), wherein the bottom surface (7.14) of the open end (7.12) has a projecting region (7.5), over which the end (10.17) of the cooling tube (10) extends, and the thickening (10.18) extends in the longitudinal direction over at least the projecting region (7.5).
 5. The arrangement as claimed in claim 4, additionally comprising an electrode holder (6) which has an elongate body (6.12) with an end (6.13) for receiving the electrode (7) and with a hollow interior (6.14), wherein the cooling tube (10) projects into the hollow interior (6.14) and at least one projection (10.6 and/or 10.7) is provided on the outer surface (10.16) of the cooling tube (10) for centring the cooling tube (10) in the electrode holder (6).
 6. The arrangement as claimed in claim 5, characterised in that a first group of projections (10.6) is provided, arranged peripherally and spaced apart from one another.
 7. The arrangement as claimed in claim 6, characterised in that a second group of projections (10.7) is provided, arranged peripherally and spaced apart from one another, wherein the second group is offset axially relative to the first group.
 8. The arrangement as claimed in claim 7, characterised in that the second group of projections (10.7) is offset peripherally relative to the first group of projections (10.6).
 9. The cooling tube (10) for an are plasma torch of claim 1, comprising: an elongate body (10.13) with a rear end (10.14) which can be releasably connected to an electrode holder (6) of an arc plasma torch and a coolant duct (10.15) extending therethrough, characterised in that for releasably connecting the rear end (10.14) to an electrode holder (6) an external thread (10.1) is provided, with a cylindrical outer surface (10.3) adjoining it for centring the cooling tube (10) relative to the electrode holder (6).
 10. The cooling tube (10) as claimed in claim 9, characterised in that a stop face (10.2) is provided for axially fixing the cooling tube (10) in the electrode holder (6).
 11. The cooling tube (10) as claimed in, claim 9 characterised in that the cylindrical outer surface (10.3) has a peripheral groove (10.4).
 12. The cooling tube (10) as claimed in claim 11, characterised in that an O-ring (10.5) is disposed in the groove (10.4) for sealing purposes.
 13. The cooling tube (10) as claimed in claim 1, characterised in that the cylindrical outer surface (10.3) has an external diameter (D10.3) which is at least the same size as or larger than the maximum external diameter (D10.1) of the external thread (10.1).
 14. An electrode holder (6) for an arc plasma torch, comprising an elongate body (6.12) with an end (6.13) for receiving an electrode (7) and a hollow interior (6.14), characterised in that in the hollow interior (6.14) an internal thread (6.1) is provided for screwing in a rear end (10.14) of a cooling tube (10), with a cylindrical inner surface (6.3) adjoining this for centring the cooling tube (10) relative to the electrode holder (6).
 15. The electrode holder (10) as claimed in claim 14, characterised in that a stop face (6.2) is provided for axially fixing the cooling tube (10) in the electrode holder (6).
 16. The electrode holder (6) as claimed in, claim 14 characterised in that the cylindrical inner surface (6.3) has an internal diameter (D6.3) which is exactly the same size as or larger than the internal diameter (D6.1) of the internal thread (6.1).
 17. The arrangement with a cooling tube (10) as claimed in claim 9 and an electrode holder (6) as claimed in, claim 9 wherein the cooling tube (10) is screwed together with the electrode holder (6) by means of the external thread (10.1) and the internal thread (6.1).
 18. The arrangement as claimed in claim 17, characterised in that the cooling tube (10) and the electrode holder (6) are designed such that towards the front end, there is an annular gap (11) between them.
 19. The arrangement as claimed in claim 17, characterised in that the cylindrical outer surface (10.3) of the cooling tube (10) and the cylindrical inner surface (6.3) of the electrode holder (6) have narrow tolerances relative to one another.
 20. The arrangement of a cooling tube (10) of claim 1 for an arc plasma torch, comprising: an elongate body (10.13) with a rear end (10.14) which can be releasably connected to an electrode holder (6) of an arc plasma torch and a coolant duct (10.15) extending therethrough, and an electrode holder (6) for an arc plasma torch, comprising an elongate body (6.12) with an end (6.13) for receiving an electrode (7) and a hollow interior (6.14), wherein on the outer surface (10.16) of the cooling tube (10) at least one projection (10.6 and/or 10.7) is provided for centring the cooling tube (10) in the electrode holder (6).
 21. The arrangement as claimed in claim 20, characterised in that a first group of projections (10.6) is provided, arranged peripherally and spaced apart from one another.
 22. The arrangement as claimed in claim 21, characterised in that a second group of projections (10.7) is provided, arranged peripherally and spaced apart from one another, wherein the second group is offset axially relative to the first group.
 23. The arrangement as claimed in claim 22, characterised in that the second group of projections (10.7) is offset peripherally relative to the first group of projections (10.6).
 24. The electrode (7) for an arc plasma torch of claim 1, comprising a hollow elongate body (7.11) with an open end (7.12) for arranging the front end of a cooling tube (10) therein and a closed end (7.13), wherein the open end has an external thread (7.4) for screwing together with the internal thread (6.4) of an electrode holder (6), characterised in that adjoining the external thread (7.4) towards the closed end (7.13), there is provided a cylindrical outer surface (7.6) for centring the electrode (7) relative to the electrode holder (6).
 25. The electrode (7) as claimed in claim 24, characterised in that a stop face (7.7) is provided for axially fixing the electrode (7) in the electrode holder (6).
 26. The electrode (7) as claimed in claim 24 characterised in that the cylindrical outer surface (7.6) has a peripheral groove (7.3).
 27. The electrode (7) as claimed in claim 26, characterised in that an O-ring (7.2) is disposed in the groove (7.3) for sealing purposes.
 28. The electrode (7) as claimed in claim 24 characterised in that the cylindrical outer surface (7.6) has an external diameter (D7.6) which is exactly the same size as or larger than the external diameter (D7.4) of the external thread (7.4).
 29. An electrode holder (6) for an arc plasma torch, comprising an elongate body (6.12) with an end (6.13), provided with an internal thread (6.4), for receiving an electrode (7), and a hollow interior (6.14), characterised in that adjoining the internal thread (6.4), there is a cylindrical inner surface (6.6) for centring the electrode (7) relative to the electrode holder (6).
 30. The electrode holder (7) as claimed in claim 29, characterised in that a stop face (6.7) is provided for axially fixing an electrode (7) in the electrode holder (6).
 31. The electrode holder (6) as claimed in claim 29 characterised in that the cylindrical inner surface (6.6) has an internal diameter (D6.6) which is exactly the same size as or larger than the internal diameter (D6.4) of the internal thread (6.4).
 32. The arrangement with an electrode (7) as claimed in claim 24 and an electrode holder (6), wherein the electrode (7) is screwed together with the electrode holder (6) by means of the external thread (7.4) and the internal thread (6.4).
 33. The arrangement as claimed in claim 32, characterised in that the cylindrical outer surface (7.6) of the electrode (7) and the cylindrical inner surface (6.6) of the electrode holder (6) have narrow tolerances relative to one another.
 34. The arc plasma torch with a cooling tube as claimed in claim 1, an electrode holder, and an electrode. 